Well Explained Question about Background

when you zoom in and out on https://www.fmcodes.com/forumthe entire forum width change, making all the icons align neatlyall the text changes according to constraint but the background stays the same, which gives it a very consistent look to the forum, very nice.like so normal:when zoomed out:

but on my forum(http://ingenmarket.forumotion.com/)I have changed my background image exactly the same dimension as fmcodes, but I do not have this consistent looknormal:when zoomed out:

the custom search bar conflicts with the text in the dropdown navbar, the background image changes size, and do not stay consistent

Question: How can I make my background image appear like the one in FMcode?